Chemically amplified negative resist composition and patterning process

ABSTRACT

A chemically amplified negative resist composition comprises a polymer comprising recurring hydroxystyrene units and recurring styrene units having electron withdrawing groups substituted thereon. In forming a pattern having a fine feature size of less than 0.1 μm, the composition exhibits a high resolution in that a resist coating formed from the composition can be processed into such a fine size pattern while the formation of bridges between pattern features is minimized.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2007-087243 filed in Japan on Mar. 29, 2007,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a chemically amplified negative resistcomposition and more particularly, to a chemically amplified negativeresist composition comprising a polymer having aromatic rings for use inprocessing of semiconductor and photomask substrates, and a patterningprocess using the same.

BACKGROUND ART

To meet the recent demand for higher integration in integrated circuits,pattern formation to a finer feature size is required. In forming resistpatterns with a feature size of 0.2 μm or less, chemically amplifiedresist compositions utilizing photo-generated acid as the catalyst aretypically used in the art because of their high sensitivity andresolution. Often, high-energy radiation such as UV, deep UV or electronbeam (EB) is used as the light source for exposure of these resistcompositions. Among others, the EB or EUV lithography is recognized mostattractive because patterns of the finest size are expectable.

Resist compositions include positive ones in which exposed areas becomesoluble and negative ones in which exposed areas are left as a pattern.A suitable composition is selected among them depending on the desiredresist pattern. In general, the chemically amplified negative resistcomposition comprises a polymer which is normally soluble in an aqueousalkaline developer, an acid generator which is decomposed to generate anacid when exposed to light, and a crosslinker which causes the polymerto crosslink in the presence of the acid serving as a catalyst, thusrendering the polymer insoluble in the developer (sometimes, thecrosslinker is incorporated in the polymer). Typically a basic compoundis added for controlling the diffusion of the acid generated upon lightexposure.

A number of negative resist compositions of the type comprising apolymer which is soluble in an aqueous alkaline developer and includesphenolic units as the alkali-soluble units were developed, especially asadapted for exposure to KrF excimer laser light. These compositions havenot been used in the ArF excimer laser lithography because the phenolicunits are not transmissive to exposure light having a wavelength of 150to 220 nm. Recently, these compositions are recognized attractive againas the negative resist composition for the EB and EUV lithographycapable of forming finer size patterns. Exemplary compositions aredescribed in JP-A 2006-201532 (corresponding to US 20060166133, EP1684118, CN 1825206) and JP-A 2006-215180.

As the required pattern size is reduced, more improvements are made onthe negative resist composition of the type using hydroxystyrene unitstypical of the phenolic units. Now that the pattern reaches a very finesize of 0.1 μm or less, there is a likelihood that the resist layer isleft like thin strings between features of a fine size pattern, which isknown as “bridge problem.” The prior art compositions fail to solve theproblem.

Also known in the art is the problem of pattern's substrate dependence,that is, the profile of a pattern alters near a processable substrate,depending on the material of which the substrate is made. As the desiredpattern size is reduced, even a minor profile alteration becomessignificant. Particularly in processing a photomask blank having theoutermost surface made of chromium oxynitride, if a pattern is formed onthe chromium oxynitride using a chemically amplified negative resistcomposition, then an “undercut” problem arises, that is, the pattern isnotched at positions in contact with the substrate. The prior artcompositions fail to fully solve the undercut problem.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a chemically amplifiednegative resist composition which can form a pattern having few bridgeswithout substantial substrate dependence, and a patterning process usingthe same.

Assuming that the cause of bridging is a low contrast of crosslinkingreaction, the inventors attempted to improve the contrast by introducinga greater number of electron donative groups into constituent units of apolymer for increasing the number of active sites in the polymer whichare reactive with a crosslinker.

In JP-A 2006-201532, cited above, the polymer used containshydroxystyrene units and carbonyloxystyrene units as styrene derivativeunits. When styrene units having substituted thereon alkoxy groups,which are electron donative groups, were used instead of thecarbonyloxystyrene units, then the number of active sites in the polymerwhich are reactive with a crosslinker could be increased withoutsignificantly altering the alkali dissolution rate of the polymer. Toverify the effect of electron donative groups, on the other hand, apolymer comprising styrene units having electron withdrawing groups wasprepared as a control. A comparison was made in resist performancebetween these polymers. Quite unexpectedly, we have found that theresist using the polymer comprising styrene units having electronwithdrawing groups substituted thereon is unlikely to leave bridges, ascompared with the prior art polymers and polymers having electrondonative groups substituted thereon, and is minimized in pattern'ssubstrate dependence.

In one aspect, the invention provides a chemically amplified negativeresist composition comprising as a base resin a polymer comprisingrecurring units having the general formulae (1) and (2):

wherein R¹ and R² are each independently hydrogen or methyl, X is anelectron withdrawing group, m is 0 or an integer of 1 to 4, and n is aninteger of 1 to 5, the polymer having a weight average molecular weightof 1,000 to 50,000. The resist composition is used to form a resistcoating which has a high resolution and gives rise to little bridgeproblem when patterned.

The electron withdrawing group represented by X has an active structuredirectly bonded to the benzene ring, examples of which include a halogenatom due to the inductive effect, and a carbonyl group, nitro group,cyano group, sulfinyl group, and sulfonyl group due to the mesomericeffect.

The most preferred examples of the electron withdrawing group includechlorine, bromine and iodine. When these elements are incorporated intoa polymer, the undercut problem of resist pattern near substrate and thebridge problem between fine pattern features are significantlymitigated.

In one preferred embodiment of the resist composition, the polymer mayfurther comprise recurring units having the general formula (3):

wherein R³ and R⁴ are each independently hydrogen, optionallysubstituted hydroxyl, or halogen, and u is 0 or an integer of 1 to 5.Inclusion of these units provides high etch resistance, enabling toreduce the thickness of resist coating.

Preferably the polymer has a weight average molecular weight (Mw) of2,000 to 8,000. With too low a Mw, the resulting pattern may be prone tothermal deformation. With too high a Mw, a bridge problem is likely tooccur during development, depending on a particular composition.

In a preferred embodiment, the polymer is obtained by removing a lowmolecular weight fraction from a polymer product as polymerized, so thatthe polymer has a dispersity Mw/Mn equal to or less than 1.7. Note thatthe dispersity is a weight average molecular weight divided by a numberaverage molecular weight, Mw/Mn, and represents a molecular weightdistribution. By removing the low molecular weight fraction so as toachieve a dispersity of 1.7 or less, the profile of a resist pattern isimproved, and especially the undercut problem associated with substratedependence is ameliorated.

In another aspect, the invention provides a pattern forming processcomprising the steps of applying the resist composition defined aboveonto a substrate to form a coating, heating the coating prior toexposure, exposing the coating to light, soft x-ray or electron beam,post-exposure heating the coating, and developing the coating with anaqueous alkaline solution.

In a further aspect, the invention provides a resist pattern formingprocess comprising the steps of providing a substrate having a surfacecomposed mainly of a transition metal compound, providing a chemicallyamplified negative resist composition comprising a polymer comprisingrecurring units having the general formulae (1) and (2) and having aweight average molecular weight of 1,000 to 50,000, and forming a resistpattern on the substrate using the chemically amplified negative resistcomposition. Typical of the material of which a photomask blank surfaceis made is a material containing a transition metal and oxygen and/ornitrogen. When a resist pattern is formed on the surface of transitionmetal compound substrate, there is a likelihood that the pattern profilebecomes defective near the substrate surface. The resist composition ofthis embodiment ensures to form a pattern of a good profile even whenapplied to the transition metal compound surface.

The electron withdrawing group represented by X has an active structuredirectly bonded to the benzene ring, examples of which include a halogenatom, carbonyl group, nitro group, cyano group, sulfinyl group, andsulfonyl group.

In a preferred embodiment of the process, the polymer may furthercomprise recurring units having the general formula (3). Inclusion ofthe units of formula (3) enables to form a thinner resist coating evenwhen the transition metal compound, which is difficult to establish aselectivity ratio during etching, is to be etched through the resist.

The transition metal compound may comprise at least one transition metalselected from chromium, molybdenum, zirconium, tantalum, tungsten,titanium, and niobium, and optionally, at least one element selectedfrom oxygen, nitrogen and carbon. These compounds are generally used asa material to form a surface layer of a photomask blank and specificallyserve as an etch mask, light-shielding film, antireflective coating orthe like.

BENEFITS OF THE INVENTION

The chemically amplified negative resist composition comprising apolymer comprising recurring hydroxystyrene units and recurring styreneunits having electron withdrawing groups substituted thereon has manyadvantages. When it is desired to form a pattern having a fine featuresize of less than 0.1 μm, the composition exhibits a high resolution inthat a resist coating formed from the composition can be processed intosuch a fine size pattern while the formation of bridges between patternfeatures is minimized.

In the processing of a photomask substrate wherein a substrate surfaceis of a transition metal compound, an undercut problem often arises in aresist pattern formed on the surface. The use of the negative resistcomposition of the invention ensures to define a resist pattern of goodprofile even on the transition metal compound because the dependence ofthe pattern on the substrate is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a resist pattern in Example 1.

FIG. 2 is a photomicrograph of a resist pattern in Comparative Example2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The polymer or high molecular weight compound used in the chemicallyamplified negative resist composition of the invention comprisesrecurring units having the general formulae (1) and (2) and has a weightaverage molecular weight of 1,000 to 50,000.

Herein R¹ and R² are each independently a hydrogen atom or methyl group,X is an electron withdrawing group, m is 0 or an integer of 1 to 4, andn is an integer of 1 to 5.

The polymer may further comprise recurring units having the generalformula (3).

Herein R³ and R⁴ are each independently a hydrogen atom, optionallysubstituted hydroxyl group, or halogen atom, and u is 0 or an integer of1 to 5.

Although the polymers used in the resist composition of the inventionmay comprise additional recurring units other than the units of formulae(1) to (3), the polymers are represented by the following generalformulae (4) and (5) provided that no additional recurring units areincluded.

Polymer I of general formula (4):

Herein R¹ and R² are each independently hydrogen or methyl, X is anelectron withdrawing group, m is 0 or an integer of 1 to 4, n is aninteger of 1 to 5, p and q are positive numbers satisfying p+q≦1.

Polymer II of general formula (5):

Herein R¹ and R² are each independently hydrogen or methyl, R³ and R⁴are each independently hydrogen, optionally substituted hydroxyl, orhalogen, X is an electron withdrawing group, m is 0 or an integer of 1to 4, n is an integer of 1 to 5, u is 0 or an integer of 1 to 5, p, qand r are positive numbers satisfying p+q+r≦1.

It is noted that the meaning of p+q+r=1 is that in a polymer comprisingrecurring units p, q, and r, the sum of recurring units p, q and r is100 mol % based on the total amount of entire recurring units. Themeaning of p+q+r<1 is that the sum of recurring units p, q, and r isless than 100 mol % based on the total amount of entire recurring units,indicating the inclusion of other recurring units.

X stands for an electron withdrawing group. The electron withdrawinggroup which is bonded to the benzene ring is effective for reducing theelectron density of the benzene ring. It may have either the inductiveeffect or the mesomeric effect. A mixture of two or more electronwithdrawing groups is acceptable. The electron withdrawing group has anactive structure directly bonded to the benzene ring, examples of whichinclude a halogen atom exhibiting the inductive effect, and a carbonylgroup, nitro group, cyano group, sulfinyl group, and sulfonyl groupexhibiting the mesomeric effect. Of these active structures, thecarbonyl, sulfinyl and sulfonyl groups are divalent and have the otherend, examples of which include optionally substituted alkyl, aryl,alkoxy, and aryloxy groups of up to 15 carbon atoms.

Specifically, suitable electron withdrawing groups X include halogenatoms, nitro groups, nitrile groups, optionally substitutedalkylcarbonyl groups of 1 to 15 carbon atoms, optionally substitutedalkoxycarbonyl groups of 1 to 15 carbon atoms, optionally substitutedarylcarbonyl groups of 7 to 20 carbon atoms, optionally substitutedaryloxycarbonyl groups of 7 to 20 carbon atoms, optionally substitutedalkylsulfinyl groups of 1 to 15 carbon atoms, optionally substitutedalkoxysulfinyl groups of 1 to 15 carbon atoms, optionally substitutedarylsulfinyl groups of 7 to 20 carbon atoms, optionally substitutedaryloxysulfinyl groups of 7 to 20 carbon atoms, optionally substitutedalkylsulfonyl groups of 1 to 15 carbon atoms, optionally substitutedalkoxysulfonyl groups of 1 to 15 carbon atoms, optionally substitutedarylsulfonyl groups of 7 to 20 carbon atoms, and optionally substitutedaryloxysulfonyl groups of 7 to 20 carbon atoms. Each of the carbonyl(—CO—), sulfinyl (—SO—), and sulfonyl (—SO₂—) moieties in the foregoinggroups is directly bonded to the benzene ring of styrene unit. Where theforegoing groups are substituted groups, exemplary substituent groupsinclude halogen, alkoxy, alkyl- or aryl-carbonyl, alkyl- oraryl-carbonyloxy, and epoxy groups.

Among others, chlorine, bromine, iodine and alkoxycarbonyl groups of thegeneral formula (6):

—CO—OR⁵  (6)

wherein R⁵ is an optionally substituted, straight, branched or cyclicalkyl group of 1 to 15 carbon atoms, are advantageous for the ease ofsynthesis and better characteristics. Exemplary straight, branched orcyclic alkyl groups represented by R⁵ include methyl, ethyl, propyl,isopropyl, n-butyl, isobutyl, tert-butyl, cyclopentyl, cyclohexyl,norbornyl, and adamantyl. Where substituted, exemplary substituentgroups include halogen, alkoxy, hydroxyl, and epoxy groups.

Of the electron withdrawing groups exemplified above, chlorine, bromine,and iodine are particularly effective in improving a pattern profile andinhibiting bridge formation.

Since the units of formula (3) are incorporated for the purpose ofimproving etch resistance as described above, R³ and R⁴ may or may nothave an additional function. Examples of optionally substituted hydroxylgroups exemplified for R³ and R⁴ include hydroxyl, alkoxy groups of 1 to15 carbon atoms, alkylcarbonyloxy groups of 2 to 15 carbon atoms,arylcarbonyloxy groups of 7 to 15 carbon atoms, alkylsulfonyloxy groupsof 1 to 15 carbon atoms, and arylsulfonyloxy groups of 6 to 15 carbonatoms.

The compositional ratio (molar ratio) of constituent units in Polymer Iis preferably selected in view of characteristics of resist material,such that p and q in formula (4) are positive numbers, and thecompositional ratio of p satisfies 0.3≦p/(p+q)≦0.9, and more preferably0.5≦p/(p+q)≦0.8. If the value of p/(p+q) is too small, fine sizepatterns cannot be formed. If the value of p/(p+q) is too large, thereis an increased likelihood of pattern collapse due to swelling.

Besides the units of formula (3), Polymer I may have furtherincorporated therein recurring units which are commonly used in resistpolymers (see JP-A 2006-201532). The acceptable compositional ratio ofrecurring units other than the units of formulae (1) to (3) is set tomeet the following requirements. In one embodiment wherein the otherrecurring units are free of alkali-soluble groups, for example,alkyl-substituted styrene units and (meth)acrylate units as disclosed inthe literature are used, the compositional ratio of recurring units offormula (1) is in a range of 30 to 90 mol %, and more preferably 50 to80 mol % of the entire recurring units. To accomplish the advantages ofthe invention, the recurring units of formula (2) must be included in anamount of at least 3 mol %, and preferably at least 5 mol % relative tothe entire recurring units. In another embodiment wherein the recurringunits other than the units of formulae (1) to (3) have alkali-solublegroups, an empirical study of previously preparing model polymers havingdifferent molar ratios and determining a compositional ratio thataffords an appropriate dissolution rate is necessary. In the otherembodiment as well, to obtain an acid-catalyzed crosslinking reactionactivity, the compositional ratio of recurring units of formula (1) ispreferably in a range of at least 30 mol %, and more preferably at least50 mol % of the entire recurring units. To accomplish the advantages ofthe invention, the recurring units of formula (2) must be included in anamount of at least 3 mol %, and preferably at least 5 mol % relative tothe entire recurring units of the polymer.

As for Polymer II, p, q and r in formula (5) are positive numbers, thecompositional ratio of p satisfies preferably 0.3≦p/(p+q+r)≦0.9, andmore preferably 0.6≦p/(p+q+r)≦0.8, and the compositional ratio of rsatisfies preferably 0<r/(p+q+r)≦0.3, and more preferably0.05≦r/(p+q+r)≦0.3. Notably the recurring units of formula (3) areincorporated for the main purpose of improving etch resistance. If thevalue of r/(p+q+r) is too large, resolution is reduced. If the value ofr/(p+q+r) is too small, the effect of improving etch resistance is notexerted.

Likewise, recurring units other than the units of formulae (1) to (3)may be incorporated in Polymer II. A number of recurring units which canconstitute polymers for use in resist compositions are known in the artas previously pointed out. The design procedure taken for incorporatingrecurring units other than the units of formulae (1) to (3) isessentially the same as in Polymer I. To accomplish the advantages ofthe invention, the recurring units of formula (2) must be included in anamount of at least 3 mol %, and preferably at least 5 mol % relative tothe entire recurring units of the polymer.

The polymers should have a weight average molecular weight (Mw) of 1,000to 50,000, preferably 2,000 to 8,000, as measured by gel permeationchromatography (GPC, HLC-8120GPC by Tosoh Corp.) versus polystyrenestandards. With too low a Mw, the resist pattern is susceptible tothermal deformation. Too high a Mw increases the tendency for a bridgingphenomenon to occur during pattern formation.

In a preferred embodiment, the polymer is obtained by removing a lowmolecular weight fraction from a polymer product as polymerized, so thatthe polymer has a dispersity Mw/Mn equal to or less than 1.7. Note thatthe dispersity is a weight average molecular weight divided by a numberaverage molecular weight, Mw/Mn, and represents a molecular weightdistribution. When a dispersity of 1.7 or less is achieved by removingthe low molecular weight fraction, the resist pattern formed on a maskblank is improved in profile, the undercut problem is significantlyameliorated, and substantial resolution is improved.

For the synthesis of the polymers, one suitable method involves feedingacetoxystyrene monomer, a styrene monomer having an electron withdrawinggroup substituted thereon, and an optional indene or other monomer to anorganic solvent, adding a radical initiator thereto, effecting thermalpolymerization, subjecting the resulting polymer to alkaline hydrolysisin the organic solvent for deprotection of acetoxy groups, thus yieldinga multi-component copolymer comprising hydroxystyrene and electronwithdrawing group-substituted styrene. Suitable organic solvents usedfor polymerization include toluene, benzene, tetrahydrofuran, diethylether, and dioxane. Suitable polymerization initiators include2,2′-azobisisobutyronitrile, 2,2′-azobis(2,4-dimethylvaleronitrile),dimethyl 2,2′-azobis(2-methyl propionate), benzoyl peroxide, and lauroylperoxide. Preferably polymerization may be effected by heating at atemperature of 40° C. to 80° C. and for a time of 2 to 100 hours, andmore preferably 5 to 40 hours. For the alkaline hydrolysis, exemplarybases are aqueous ammonia and triethylamine; the reaction temperature is−20° C. to 100° C., and preferably 0° C. to 60° C.; and the time is 0.2to 100 hours, and preferably 0.5 to 40 hours.

The polymer product obtained by the abovementioned polymerization methodmay be adjusted to a dispersity of 1.7 or less by dissolving the polymerproduct in a good solvent, admitting the polymer solution into a badsolvent with stirring, and fractionating off the low molecular weightfraction in the bad solvent. Examples of the good and bad solvents usedin this fractionation step include acetone, ethyl acetate, methylacetate, propylene glycol monomethyl ether, propylene glycol monoethylether, propylene glycol methyl ether acetate, propylene glycol ethylether acetate, tetrahydrofuran, diethyl ether, water, methanol, ethanol,isopropanol, hexane, pentane, toluene, and benzene. Of these solvents, achoice may be made depending on whether the polymer subject tofractionation is lipophilic or hydrophilic. Other fractionation methodsinclude precipitation of a polymer in a bad solvent, and separation intotwo layers of a good solvent (containing a polymer component to becollected) and a bad solvent (containing a low molecular weight fractionto be removed).

It is understood that the synthesis method is not limited to theforegoing.

Photoacid Generator

While the aforementioned polymer is compounded as a base resin in achemically amplified negative resist composition, an acid generatorwhich is decomposed to generate an acid upon exposure to high-energyradiation, referred to as “photoacid generator,” may be compounded aswell. It is noted that an acid generator which is sensitive to EBexposure is also referred to as “photoacid generator” in a sense todistinguish from a thermal acid generator capable of generating an acidby heat. A number of photoacid generators are known in the art includingthose described in JP-A 2006-201532 and JP-A 2006-215180 cited above.Generally, any of well-known photoacid generators may be used herein.

Suitable photoacid generators include sulfonium salts, iodonium salts,sulfonyldiazomethane and N-sulfonyloxyimide photoacid generators.Exemplary photoacid generators are given below while they may be usedalone or in admixture of two or more.

Sulfonium salts are salts of sulfonium cations with sulfonate anions.Exemplary sulfonium cations include triphenylsulfonium,(4-tert-butoxyphenyl)diphenylsulfonium,bis(4-tert-butoxyphenyl)phenylsulfonium,tris(4-tert-butoxyphenyl)sulfonium,(3-tert-butoxyphenyl)diphenylsulfonium,bis(3-tert-butoxyphenyl)phenylsulfonium,tris(3-tert-butoxyphenyl)sulfonium,(3,4-di-tert-butoxyphenyl)diphenylsulfonium,bis(3,4-di-tert-butoxyphenyl)phenylsulfonium,tris(3,4-di-tert-butoxyphenyl)sulfonium,diphenyl(4-thiophenoxyphenyl)sulfonium,(4-tert-butoxycarbonylmethyloxyphenyl)diphenylsulfonium,tris(4-tert-butoxycarbonylmethyloxyphenyl)sulfonium,(4-tert-butoxyphenyl)bis(4-dimethylaminophenyl)sulfonium,tris(4-dimethylaminophenyl)sulfonium, 2-naphthyldiphenylsulfonium,dimethyl-2-naphthylsulfonium, 4-hydroxyphenyldimethylsulfonium,4-methoxyphenyldimethylsulfonium, trimethylsulfonium,2-oxocyclohexylcyclohexylmethylsulfonium, trinaphthylsulfonium, andtribenzylsulfonium. Exemplary sulfonate anions includetrifluoromethanesulfonate, nonafluorobutanesulfonate,heptadecafluorooctanesulfonate, 2,2,2-trifluoroethanesulfonate,pentafluorobenzenesulfonate, 4-trifluoromethylbenzenesulfonate,4-fluorobenzenesulfonate, toluenesulfonate, benzenesulfonate,4-(4-toluenesulfonyloxy)benzenesulfonate, naphthalenesulfonate,camphorsulfonate, octanesulfonate, dodecylbenzenesulfonate,butanesulfonate, and methanesulfonate. Sulfonium salts based oncombination of the foregoing examples are included.

Iodinium salts are salts of iodonium cations with sulfonate anions.Exemplary iodonium cations are aryliodonium cations includingdiphenyliodinium, bis(4-tert-butylphenyl)iodonium,4-tert-butoxyphenylphenyliodonium, and 4-methoxyphenylphenyliodonium.Exemplary sulfonate anions include trifluoromethanesulfonate,nonafluorobutanesulfonate, heptadecafluorooctanesulfonate,2,2,2-trifluoroethanesulfonate, pentafluorobenzenesulfonate,4-trifluoromethylbenzenesulfonate, 4-fluorobenzenesulfonate,toluenesulfonate, benzenesulfonate,4-(4-toluenesulfonyloxy)benzenesulfonate, naphthalenesulfonate,camphorsulfonate, octanesulfonate, dodecylbenzenesulfonate,butanesulfonate, and methanesulfonate. Iodonium salts based oncombination of the foregoing examples are included.

Exemplary sulfonyldiazomethane compounds include bissulfonyldiazomethanecompounds and sulfonyl-carbonyldiazomethane compounds such asbis(ethylsulfonyl)diazomethane, bis(1-methylpropylsulfonyl)diazomethane,bis(2-methylpropylsulfonyl)diazomethane,bis(1,1-dimethylethylsulfonyl)diazomethane,bis(cyclohexylsulfonyl)diazomethane,bis(perfluoroisopropylsulfonyl)diazomethane,bis(phenylsulfonyl)diazomethane,bis(4-methylphenylsulfonyl)diazomethane,bis(2,4-dimethylphenylsulfonyl)diazomethane,bis(2-naphthylsulfonyl)diazomethane,4-methylphenylsulfonylbenzoyldiazomethane,tert-butylcarbonyl-4-methylphenylsulfonyldiazomethane,2-naphthylsulfonylbenzoyldiazomethane,4-methylphenylsulfonyl-2-naphthoyldiazomethane,methylsulfonylbenzoyldiazomethane, andtert-butoxycarbonyl-4-methylphenylsulfonyldiazomethane.

N-sulfonyloxyimide photoacid generators include combinations of imideskeletons with sulfonate skeletons. Exemplary imide skeletons aresuccinimide, naphthalene dicarboxylic acid imide, phthalimide,cyclohexyldicarboxylic acid imide, 5-norbornene-2,3-dicarboxylic acidimide, and 7-oxabicyclo[2.2.1]-5-heptene-2,3-dicarboxylic acid imide.Exemplary sulfonate skeletons include trifluoromethanesulfonate,nonafluorobutanesulfonate, heptadecafluorooctanesulfonate,2,2,2-trifluoroethanesulfonate, pentafluorobenzenesulfonate,4-trifluoromethylbenzenesulfonate, 4-fluorobenzenesulfonate,toluenesulfonate, benzenesulfonate, naphthalenesulfonate,camphorsulfonate, octanesulfonate, dodecylbenzenesulfonate,butanesulfonate, and methanesulfonate.

Additionally, other photoacid generators as listed below are useful.Benzoinsulfonate photoacid generators include benzoin tosylate, benzoinmesylate, and benzoin butanesulfonate.

Pyrogallol trisulfonate photoacid generators include pyrogallol,fluoroglycine, catechol, resorcinol, hydroquinone, in which all thehydroxyl groups are substituted with sulfonate groups such astrifluoromethanesulfonate, nonafluorobutanesulfonate,heptadecafluorooctanesulfonate, 2,2,2-trifluoroethanesulfonate,pentafluorobenzenesulfonate, 4-trifluoromethylbenzenesulfonate,4-fluorobenzenesulfonate, toluenesulfonate, benzenesulfonate,naphthalenesulfonate, camphorsulfonate, octanesulfonate,dodecylbenzenesulfonate, butanesulfonate, and methanesulfonate.

Nitrobenzyl sulfonate photoacid generators include 2,4-dinitrobenzylsulfonate, 2-nitrobenzyl sulfonate, and 2,6-dinitrobenzyl sulfonate,with exemplary sulfonates including trifluoromethanesulfonate,nonafluorobutanesulfonate, heptadecafluorooctanesulfonate,2,2,2-trifluoroethanesulfonate, pentafluorobenzenesulfonate,4-trifluoromethylbenzenesulfonate, 4-fluorobenzenesulfonate,toluenesulfonate, benzenesulfonate, naphthalenesulfonate,camphorsulfonate, octanesulfonate, dodecylbenzenesulfonate,butanesulfonate, and methanesulfonate. Also useful are analogousnitrobenzyl sulfonate compounds in which the nitro group on the benzylside is substituted with a trifluoromethyl group.

Sulfone photoacid generators include bis(phenylsulfonyl)methane,bis(4-methylphenylsulfonyl)methane, bis(2-naphthylsulfonyl)methane,2,2-bis(phenylsulfonyl)propane, 2,2-bis(4-methylphenylsulfonyl)propane,2,2-bis(2-naphthylsulfonyl)propane,2-methyl-2-(p-toluenesulfonyl)propiophenone,2-cyclohexylcarbonyl-2-(p-toluenesulfonyl)propane, and2,4-dimethyl-2-(p-toluenesulfonyl)pentan-3-one.

Photoacid generators in the form of glyoxime derivatives includebis-O-(p-toluenesulfonyl)-α-dimethylglyoxime,bis-O-(p-toluenesulfonyl)-α-diphenylglyoxime,bis-O-(p-toluenesulfonyl)-α-dicyclohexylglyoxime,bis-O-(p-toluenesulfonyl)-2,3-pentanedioneglyoxime,bis-O-(p-toluenesulfonyl)-2-methyl-2,3-pentanedioneglyoxime,bis-O-(n-butanesulfonyl)-α-dimethylglyoxime,bis-O-(n-butanesulfonyl)-α-diphenylglyoxime,bis-O-(n-butanesulfonyl)-α-dicyclohexylglyoxime,bis-O-(n-butanesulfonyl)-2,3-pentanedioneglyoxime,bis-O-(n-butanesulfonyl)-2-methyl-3,4-pentanedioneglyoxime,bis-O-(methanesulfonyl)-α-dimethylglyoxime,bis-O-(trifluoromethanesulfonyl)-α-dimethylglyoxime,bis-O-(1,1,1-trifluoroethanesulfonyl)-α-dimethylglyoxime,bis-O-(tert-butanesulfonyl)-α-dimethylglyoxime,bis-O-(perfluorooctanesulfonyl)-α-dimethylglyoxime,bis-O-(cyclohexylsulfonyl)-α-dimethylglyoxime,bis-O-(benzenesulfonyl)-α-dimethylglyoxime,bis-O-(p-fluorobenzenesulfonyl)-α-dimethylglyoxime,bis-O-(p-tert-butylbenzenesulfonyl)-α-dimethylglyoxime,bis-O-(xylenesulfonyl)-α-dimethylglyoxime, andbis-O-(camphorsulfonyl)-α-dimethylglyoxime.

Of these, sulfonium salt, bissulfonyldiazomethane and N-sulfonyloxyimidephotoacid generators are preferred.

While the anion of an optimum acid generated varies depending on thereactivity of crosslinker in the resist composition, it is generallyselected from those anions which are nonvolatile and not extremelydiffusive. Suitable anions include benzenesulfonate, toluenesulfonate,4-(4-toluenesulfonyloxy)benzenesulfonate, pentafluorobenzenesulfonate,2,2,2-trifluoroethanesulfonate, nonafluorobutanesulfonate,heptadecafluorooctanesulfonate, and camphorsulfonate anions.

In the negative resist composition of the invention, the photoacidgenerator is preferably added in an amount of 0 to 30 parts by weight,more preferably 2 to 20 parts by weight per 100 parts by weight of thepolymer or base resin. The photoacid generators may be used alone or inadmixture of two or more. The transmittance of the resist film can becontrolled by using a photoacid generator having a low transmittance atthe exposure wavelength and adjusting the amount of the photoacidgenerator added.

Crosslinker

A crosslinker is an essential component in the chemically amplifiednegative resist composition. In some cases, the crosslinker can beincorporated in the polymer, for example, by adding epoxy group-bearingunits to the units of formulae (1) to (3) during polymerization.Usually, crosslinking compounds as described below are separately addedto the composition.

The crosslinker used herein may be any of crosslinkers which react withthe polymer to induce intramolecular and intermolecular crosslinkageunder the catalysis of the acid generated by the photoacid generator.Typically they are compounds having a plurality of functional groupswhich undergo electrophilic reaction with recurring units of formula (1)in the polymer to form bonds therewith. A number of such compounds arealready known (see JP-A 2006-201532 and JP-A 2006-215180).

The crosslinker used in the resist composition may be any of well-knowncrosslinkers. Suitable crosslinkers include alkoxymethylglycolurils andalkoxymethylmelamines. Examples of suitable alkoxymethylglycolurilsinclude tetramethoxymethylglycoluril,1,3-bismethoxymethyl-4,5-bismethoxyethylene urea, and bismethoxymethylurea. Examples of suitable alkoxymethylmelamines includehexamethoxymethylmelamine and hexaethoxymethylmelamine.

In the negative resist composition of the invention, the crosslinker ispreferably added in an amount of 2 to 40 parts by weight, morepreferably 5 to 20 parts by weight per 100 parts by weight of the baseresin. The crosslinkers may be used alone or in admixture of two ormore. The transmittance of the resist film can be controlled by using acrosslinker having a low transmittance at the exposure wavelength andadjusting the amount of the crosslinker added.

Basic Compound

To the chemically amplified negative resist composition, a basiccompound may be added as a component capable of controlling thediffusion distance of acid. Controlling the diffusion distance leads tobetter resolution, reduces the substrate and environment dependence, andimproves the exposure latitude and pattern profile.

Examples of basic compounds include primary, secondary, and tertiaryaliphatic amines, mixed amines, aromatic amines, heterocyclic amines,nitrogen-containing compounds having carboxyl group, nitrogen-containingcompounds having sulfonyl group, nitrogen-containing compounds havinghydroxyl group, nitrogen-containing compounds having hydroxyphenylgroup, alcoholic nitrogen-containing compounds, amide derivatives, andimide derivatives.

Examples of suitable primary aliphatic amines include ammonia,methylamine, ethylamine, n-propylamine, isopropylamine, n-butylamine,isobutylamine, sec-butylamine, tert-butylamine, pentylamine,tert-amylamine, cyclopentylamine, hexylamine, cyclohexylamine,heptylamine, octylamine, nonylamine, decylamine, dodecylamine,cetylamine, methylenediamine, ethylenediamine, andtetraethylenepentamine. Examples of suitable secondary aliphatic aminesinclude dimethylamine, diethylamine, di-n-propylamine, diisopropylamine,di-n-butylamine, diisobutylamine, di-sec-butylamine, dipentylamine,dicyclopentylamine, dihexylamine, dicyclohexylamine, diheptylamine,dioctylamine, dinonylamine, didecylamine, didodecylamine, dicetylamine,N,N-dimethylmethylenediamine, N,N-dimethylethylenediamine, andN,N-dimethyltetraethylenepentamine. Examples of suitable tertiaryaliphatic amines include trimethylamine, triethylamine,tri-n-propylamine, triisopropylamine, tri-n-butylamine,triisobutylamine, tri-sec-butylamine, tripentylamine,tricyclopentylamine, trihexylamine, tricyclohexylamine, triheptylamine,trioctylamine, trinonylamine, tridecylamine, tridodecylamine,tricetylamine, N,N,N′,N′-tetramethylmethylenediamine,N,N,N′,N′-tetramethylethylenediamine, andN,N,N′,N′-tetramethyltetraethylenepentamine.

Examples of suitable mixed amines include dimethylethylamine,methylethylpropylamine, benzylamine, phenethylamine, andbenzyldimethylamine. Examples of suitable aromatic and heterocyclicamines include aniline derivatives (e.g., aniline, N-methylaniline,N-ethylaniline, N-propylaniline, N,N-dimethylaniline, 2-methylaniline,3-methylaniline, 4-methylaniline, ethylaniline, propylaniline,trimethylaniline, 2-nitroaniline, 3-nitroaniline, 4-nitroaniline,2,4-dinitroaniline, 2,6-dinitroaniline, 3,5-dinitroaniline, andN,N-dimethyltoluidine), diphenyl(p-tolyl)amine, methyldiphenylamine,triphenylamine, phenylenediamine, naphthylamine, diaminonaphthalene,pyrrole derivatives (e.g., pyrrole, 2H-pyrrole, 1-methylpyrrole,2,4-dimethylpyrrole, 2,5-dimethylpyrrole, and N-methylpyrrole), oxazolederivatives (e.g., oxazole and isooxazole), thiazole derivatives (e.g.,thiazole and isothiazole), imidazole derivatives (e.g., imidazole,4-methylimidazole, and 4-methyl-2-phenylimidazole), pyrazolederivatives, furazan derivatives, pyrroline derivatives (e.g., pyrrolineand 2-methyl-1-pyrroline), pyrrolidine derivatives (e.g., pyrrolidine,N-methylpyrrolidine, pyrrolidinone, and N-methylpyrrolidone),imidazoline derivatives, imidazolidine derivatives, pyridine derivatives(e.g., pyridine, methylpyridine, ethylpyridine, propylpyridine,butylpyridine, 4-(1-butylpentyl)pyridine, dimethylpyridine,trimethylpyridine, triethylpyridine, phenylpyridine,3-methyl-2-phenylpyridine, 4-tert-butylpyridine, diphenylpyridine,benzylpyridine, methoxypyridine, butoxypyridine, dimethoxypyridine,1-methyl-2-pyridine, 4-pyrrolidinopyridine, 1-methyl-4-phenylpyridine,2-(1-ethylpropyl)pyridine, aminopyridine, and dimethylaminopyridine),pyridazine derivatives, pyrimidine derivatives, pyrazine derivatives,pyrazoline derivatives, pyrazolidine derivatives, piperidinederivatives, piperazine derivatives, morpholine derivatives, indolederivatives, isoindole derivatives, 1H-indazole derivatives, indolinederivatives, quinoline derivatives (e.g., quinoline and3-quinolinecarbonitrile), isoquinoline derivatives, cinnolinederivatives, quinazoline derivatives, quinoxaline derivatives,phthalazine derivatives, purine derivatives, pteridine derivatives,carbazole derivatives, phenanthridine derivatives, acridine derivatives,phenazine derivatives, 1,10-phenanthroline derivatives, adeninederivatives, adenosine derivatives, guanine derivatives, guanosinederivatives, uracil derivatives, and uridine derivatives.

Examples of suitable nitrogen-containing compounds with carboxyl groupinclude aminobenzoic acid, indolecarboxylic acid, and amino acidderivatives (e.g. nicotinic acid, alanine, alginine, aspartic acid,glutamic acid, glycine, histidine, isoleucine, glycylleucine, leucine,methionine, phenylalanine, threonine, lysine,3-aminopyrazine-2-carboxylic acid, and methoxyalanine). Examples ofsuitable nitrogen-containing compounds with sulfonyl group include3-pyridinesulfonic acid and pyridinium p-toluenesulfonate. Examples ofsuitable nitrogen-containing compounds with hydroxyl group,nitrogen-containing compounds with hydroxyphenyl group, and alcoholicnitrogen-containing compounds include 2-hydroxypyridine, aminocresol,2,4-quinolinediol, 3-indolemethanol hydrate, monoethanolamine,diethanolamine, triethanolamine, N-ethyldiethanolamine,N,N-diethylethanolamine, triisopropanolamine, 2,2′-iminodiethanol,2-aminoethanol, 3-amino-1-propanol, 4-amino-1-butanol,4-(2-hydroxyethyl)morpholine, 2-(2-hydroxyethyl)pyridine,1-(2-hydroxyethyl)piperazine, 1-[2-(2-hydroxyethoxy)ethyl]piperazine,piperidine ethanol, 1-(2-hydroxyethyl)pyrrolidine,1-(2-hydroxyethyl)-2-pyrrolidinone, 3-piperidino-1,2-propanediol,3-pyrrolidino-1,2-propanediol, 8-hydroxyjulolidine, 3-quinuclidinol,3-tropanol, 1-methyl-2-pyrrolidine ethanol, 1-aziridine ethanol,N-(2-hydroxyethyl)phthalimide, and N-(2-hydroxyethyl)isonicotinamide.

Examples of suitable amide derivatives include formamide,N-methylformamide, N,N-dimethylformamide, acetamide, N-methylacetamide,N,N-dimethylacetamide, propionamide, and benzamide. Suitable imidederivatives include phthalimide, succinimide, and maleimide.

In addition, one or more of basic compounds of the following generalformula (B)-1 may also be included.

N(Z)_(n)(Y)_(3-n)  (B)-1

In the formula, n is equal to 1, 2 or 3; Y is independently hydrogen ora straight, branched or cyclic alkyl group of 1 to 20 carbon atoms whichmay contain a hydroxyl group or ether group; and Z is independentlyselected from groups of the following general formulas (Z)-1 to (Z)-3,and two or three Z may bond together to form a ring.

In the formulas, R³⁰⁰, R³⁰² and R³⁰⁵ are independently straight orbranched C₁-C₄ alkylene groups; R³⁰¹ and R³⁰⁴ are independently hydrogenor straight, branched or cyclic C₁-C₂₀ alkyl groups, which may containat least one hydroxyl group, ether group, ester group or lactone ring;R³⁰³ is a single bond or a straight or branched C₁-C₄ alkylene group;and R³⁰⁶ is a straight, branched or cyclic C₁-C₂₀ alkyl group, which maycontain at least one hydroxyl group, ether group, ester group or lactonering.

Illustrative examples of the basic compounds of formula (B)-1 include,but are not limited to, tris[(2-methoxymethoxy)ethyl]amine,tris[2-(2-methoxyethoxy)ethyl]amine,tris[2-(2-methoxyethoxymethoxy)ethyl]amine,tris[2-(1-methoxyethoxy)ethyl]amine, tris[2-(1-ethoxyethoxy)ethyl]amine,tris[2-(1-ethoxypropoxy)ethyl]amine,tris[2-(2-(2-hydroxyethoxy)ethoxy)ethyl]amine,4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane,4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane,1,4,10,13-tetraoxa-7,16-diazabicyclooctadecane,1-aza-12-crown-4,1-aza-15-crown-5,1-aza-18-crown-6,tris(2-formyloxyethyl)amine, tris(2-acetoxyethyl)amine,tris(2-propionyloxyethyl)amine, tris(2-butyryloxyethyl)amine,tris(2-isobutyryloxyethyl)amine, tris(2-valeryloxyethyl)amine,tris(2-pivaloyloxyethyl)amine,N,N-bis(2-acetoxyethyl)-2-(acetoxyacetoxy)ethylamine,tris(2-methoxycarbonyloxyethyl)amine,tris(2-tert-butoxycarbonyloxyethyl)amine,tris[2-(2-oxopropoxy)ethyl]amine,tris[2-(methoxycarbonylmethyl)oxyethyl]amine,tris[2-(tert-butoxycarbonylmethyloxy)ethyl]amine,tris[2-(cyclohexyloxycarbonylmethyloxy)ethyl]amine,tris(2-methoxycarbonylethyl)amine, tris(2-ethoxycarbonylethyl)amine,N,N-bis(2-hydroxyethyl)-2-(methoxycarbonyl)ethylamine,N,N-bis(2-acetoxyethyl)-2-(methoxycarbonyl)ethylamine,N,N-bis(2-hydroxyethyl)-2-(ethoxycarbonyl)ethylamine,N,N-bis(2-acetoxyethyl)-2-(ethoxycarbonyl)ethylamine,N,N-bis(2-hydroxyethyl)-2-(2-methoxyethoxycarbonyl)ethylamine,N,N-bis(2-acetoxyethyl)-2-(2-methoxyethoxycarbonyl)ethylamine,N,N-bis(2-hydroxyethyl)-2-(2-hydroxyethoxycarbonyl)ethylamine,N,N-bis(2-acetoxyethyl)-2-(2-acetoxyethoxycarbonyl)ethylamine,N,N-bis(2-hydroxyethyl)-2-[(methoxycarbonyl)methoxycarbonyl]-ethylamine,N,N-bis(2-acetoxyethyl)-2-[(methoxycarbonyl)methoxycarbonyl]-ethylamine,N,N-bis(2-hydroxyethyl)-2-(2-oxopropoxycarbonyl)ethylamine,N,N-bis(2-acetoxyethyl)-2-(2-oxopropoxycarbonyl)ethylamine,N,N-bis(2-hydroxyethyl)-2-(tetrahydrofurfuryloxycarbonyl)-ethylamine,N,N-bis(2-acetoxyethyl)-2-(tetrahydrofurfuryloxycarbonyl)-ethylamine,N,N-bis(2-hydroxyethyl)-2-[(2-oxotetrahydrofuran-3-yl)oxy-carbonyl]ethylamine,N,N-bis(2-acetoxyethyl)-2-[(2-oxotetrahydrofuran-3-yl)oxy-carbonyl]ethylamine,N,N-bis(2-hydroxyethyl)-2-(4-hydroxybutoxycarbonyl)ethylamine,N,N-bis(2-formyloxyethyl)-2-(4-formyloxybutoxycarbonyl)-ethylamine,N,N-bis(2-formyloxyethyl)-2-(2-formyloxyethoxycarbonyl)-ethylamine,N,N-bis(2-methoxyethyl)-2-(methoxycarbonyl)ethylamine,N-(2-hydroxyethyl)-bis[2-(methoxycarbonyl)ethyl]amine,N-(2-acetoxyethyl)-bis[2-(methoxycarbonyl)ethyl]amine,N-(2-hydroxyethyl)-bis[2-(ethoxycarbonyl)ethyl]amine,N-(2-acetoxyethyl)-bis[2-(ethoxycarbonyl)ethyl]amine,N-(3-hydroxy-1-propyl)-bis[2-(methoxycarbonyl)ethyl]amine,N-(3-acetoxy-1-propyl)-bis[2-(methoxycarbonyl)ethyl]amine,N-(2-methoxyethyl)-bis[2-(methoxycarbonyl)ethyl]amine,N-butyl-bis[2-(methoxycarbonyl)ethyl]amine,N-butyl-bis[2-(2-methoxyethoxycarbonyl)ethyl]amine,N-methyl-bis(2-acetoxyethyl)amine, N-ethyl-bis(2-acetoxyethyl)amine,N-methyl-bis(2-pivaloyloxyethyl)amine,N-ethyl-bis[2-(methoxycarbonyloxy)ethyl]amine,N-ethyl-bis[2-(tert-butoxycarbonyloxy)ethyl]amine,tris(methoxycarbonylmethyl)amine, tris(ethoxycarbonylmethyl)amine,N-butyl-bis(methoxycarbonylmethyl)amine,N-hexyl-bis(methoxycarbonylmethyl)amine, andβ-(diethylamino)-δ-valerolactone.

The basic compounds may be used alone or in admixture of two or more.The basic compound is preferably formulated in an amount of 0 to 2parts, and especially 0.01 to 1 part by weight, per 100 parts by weightof the base resin in the resist composition. The use of more than 2parts of the basis compound may result in too low a sensitivity.

Organic Solvent

In order that the negative resist composition be coated to form a resistfilm, the foregoing components are dissolved in an organic solvent toformulate the composition in solution form. A number of organic solventsare known and used to this end. Illustrative, non-limiting, examples ofsuitable organic solvents include butyl acetate, amyl acetate,cyclohexyl acetate, 3-methoxybutyl acetate, methyl ethyl ketone, methylamyl ketone, cyclohexanone, cyclopentanone, 3-ethoxyethyl propionate,3-ethoxymethyl propionate, 3-methoxymethyl propionate, methylacetoacetate, ethyl acetoacetate, diacetone alcohol, methyl pyruvate,ethyl pyruvate, propylene glycol monomethyl ether, propylene glycolmonoethyl ether, propylene glycol monomethyl ether propionate, propyleneglycol monoethyl ether propionate, ethylene glycol monomethyl ether,ethylene glycol monoethyl ether, diethylene glycol monomethyl ether,diethylene glycol monoethyl ether, 3-methyl-3-methoxybutanol,N-methylpyrrolidone, dimethyl sulfoxide, γ-butyrolactone, propyleneglycol alkyl ether acetates such as propylene glycol methyl etheracetate, propylene glycol ethyl ether acetate, and propylene glycolpropyl ether acetate, alkyl lactates such as methyl lactate, ethyllactate, and propyl lactate, and tetramethylene sulfone.

Of these, the propylene glycol alkyl ether acetates and alkyl lactatesare especially preferred. The solvents may be used alone or in admixtureof two or more. An exemplary useful solvent mixture is a mixture ofpropylene glycol alkyl ether acetates and/or alkyl lactates. It is notedthat the alkyl groups of the propylene glycol alkyl ether acetates arepreferably those of 1 to 4 carbon atoms, for example, methyl, ethyl andpropyl, with methyl and ethyl being especially preferred. Since thepropylene glycol alkyl ether acetates include 1,2- and 1,3-substitutedones, each includes three isomers depending on the combination ofsubstituted positions, which may be used alone or in admixture. It isalso noted that the alkyl groups of the alkyl lactates are preferablythose of 1 to 4 carbon atoms, for example, methyl, ethyl and propyl,with methyl and ethyl being especially preferred.

When the propylene glycol alkyl ether acetate is used as the solvent, itpreferably accounts for at least 50% by weight of the entire solvent.Also when the alkyl lactate or propylene glycol alkyl ether is used asthe solvent, it preferably accounts for at least 50% by weight of theentire solvent. When a mixture of propylene glycol alkyl ether acetateand alkyl lactate or propylene glycol alkyl ether is used as thesolvent, that mixture preferably accounts for at least 50% by weight ofthe entire solvent. In this solvent mixture, it is further preferredthat the propylene glycol alkyl ether acetate is 5 to 40% by weight andthe alkyl lactate or propylene glycol alkyl ether is 60 to 95% byweight. A lower proportion of the propylene glycol alkyl ether acetatewould invite a problem of inefficient coating whereas a higherproportion thereof would provide insufficient dissolution and allow forparticle and foreign matter formation. A lower proportion of the alkyllactate or propylene glycol alkyl ether would provide insufficientdissolution and cause the problem of many particles and foreign matterwhereas a higher proportion thereof would lead to a composition whichhas a too high viscosity to apply and loses storage stability.

In the negative resist composition, the solvent is preferably used in anamount of 300 to 2,000 parts by weight, especially 400 to 1,000 parts byweight per 100 parts by weight of the base resin. The concentration ofthe resulting composition is not limited thereto as long as a film canbe formed by existing methods.

Surfactant

To the chemically amplified negative resist composition of theinvention, a surfactant may be added for improving coatingcharacteristics or the like.

Illustrative, non-limiting, examples of the surfactant include nonionicsurfactants, for example, polyoxyethylene alkyl ethers such aspolyoxyethylene lauryl ether, polyoxyethylene stearyl ether,polyoxyethylene cetyl ether, and polyoxyethylene oleyl ether,polyoxyethylene alkylaryl ethers such as polyoxyethylene octylphenolether and polyoxyethylene nonylphenol ether, polyoxyethylenepolyoxypropylene block copolymers, sorbitan fatty acid esters such assorbitan monolaurate, sorbitan monopalmitate, and sorbitan monostearate,and polyoxyethylene sorbitan fatty acid esters such as polyoxyethylenesorbitan monolaurate, polyoxyethylene sorbitan monopalmitate,polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitantrioleate, and polyoxyethylene sorbitan tristearate; fluorochemicalsurfactants such as EFTOP EF301, EF303 and EF352 (Tohkem Products Co.,Ltd.), Megaface F171, F172 and F173 (Dainippon Ink & Chemicals, Inc.),Fluorad FC430 and FC431 (Sumitomo 3M Co., Ltd.), Asahiguard AG710,Surflon S-381, S-382, SC101, SC102, SC103, SC104, SC105, SC106, SurfynolE1004, KH-10, KH-20, KH-30 and KH-40 (Asahi Glass Co., Ltd.);organosiloxane polymers KP341, X-70-092 and X-70-093 (Shin-Etsu ChemicalCo., Ltd.), acrylic acid or methacrylic acid Polyflow No. 75 and No. 95(Kyoeisha Ushi Kagaku Kogyo K.K.). Inter alia, Fluorad FC430, SurflonS-381, Surfynol E1004, KH-20 and KH-30 are preferred. These surfactantsmay be used alone or in admixture.

In the chemically amplified negative resist composition of theinvention, the surfactant is preferably formulated in an amount of up to2 parts, and especially up to 1 part by weight, per 100 parts by weightof the base resin.

Process

A resist pattern is formed from the chemically amplified negative resistcomposition of the invention by any ordinary lithography processincluding coating step of the resist composition onto a processablesubstrate (or substrate to be processed), pattern-wise exposure stepusing high-energy radiation, and development step using an aqueousalkaline developer.

The material of which the processable substrate or its outermost surfacelayer is made is not particularly limited. In the case of semiconductorwafers, for example, silicon wafers may be used, and examples of theoutermost surface layer include Si, SiO₂, SiN, SiON, TiN, WSi, BPSG,SOG, and organic antireflective films.

In another embodiment, a resist pattern is formed on a photomask blank,from which a photomask is obtained. Typical transparent substrates usedherein include transparent substrates of quartz and calcium fluoride. Inmost cases, necessary functional films such as a light-shielding film,antireflective coating, phase shift film, and optionally, etch stop filmand etch mask film are laid in sequence on the substrate, depending onthe intended application. In some special cases, such a functional filmis omitted. Examples of the material of which the functional film ismade include silicon, or transition metals such as chromium, molybdenum,zirconium, tantalum, tungsten, titanium and niobium, which may be usedto form a layer. Examples of the material of which the outermost surfacelayer is made include materials mainly containing silicon or silicon andoxygen and/or nitrogen, silicon compound materials mainly containing atransition metal in addition to the foregoing, and transition metalcompound materials mainly containing a transition metal, specifically atleast one of chromium, molybdenum, zirconium, tantalum, tungsten,titanium, and niobium, and optionally at least one of oxygen, nitrogen,and carbon.

Of these materials, the transition metal compound material tends to giverise to the undercut problem. Specifically, a photomask blank includesan outermost surface layer of a transition metal compound material,specifically a transition metal compound material containing oxygenand/or nitrogen, and more specifically a transition metal compoundmaterial containing chromium and oxygen and/or nitrogen. When a patternis formed on this photomask blank using a chemically amplified negativeresist composition, the pattern tends to be constricted near thesubstrate, resulting in a so-called “undercut” shape. Nevertheless, thechemically amplified negative resist composition of the invention issuccessful in ameliorating the undercut problem, as compared with priorart resist compositions. Thus the pattern forming process of theinvention is advantageous.

The process starts with a coating step. In applying the inventive resistcomposition, any of well-known application techniques including spincoating, roll coating, flow coating, dip coating, spray coating, anddoctor coating may be used. Spin coating is preferred for consistentformation of a thin coating.

The thickness of the coating is selected depending on the minimum linewidth of the desired pattern and the etching rate of the processablesubstrate. Usually a thickness which is equal to or greater than theminimum line width by a factor of 1 to 8 is selected.

The resist coating is then heated (i.e., prebaked) on a hot plate,heating furnace or the like for removing the unnecessary organic solventremaining in the resist coating. The heating conditions, which vary withthe type of substrate, may not be determined unequivocally. Where a hotplate is used, typical prebaking conditions include a temperature of 60to 150° C. for about 1 to 10 minutes, preferably 80 to 120° C. for about1 to 5 minutes.

The pattern exposure step is imagewise exposure in a well-known wayusing high-energy radiation providing a high transmittance to thebenzene ring, for example, deep-UV having a wavelength equal to or morethan 230 nm, typically KrF excimer laser radiation, EB, EUV, and X-ray.In the processing of a photomask blank, EB exposure is always used. Forthe EB exposure, an exposure dose of about 0.1 to 20 μC/cm² ispreferred, with an exposure dose of about 3 to 10 μC/cm² being morepreferred.

After the pattern exposure, the coated substrate is heated again (orpost-exposure baked) for promoting acid-catalyzed crosslinking reaction.Where a hot plate is used, for example, the exposed areas of the coatingare appropriately cured by heating at 60 to 150° C. for about 1 to 20minutes, preferably at 80 to 120° C. for about 1 to 10 minutes.

In the subsequent development step, an aqueous alkaline developer isused to dissolving away the unexposed areas of the coating, leaving thedesired resist pattern. Development is typically carried out in anaqueous solution of 0.1 to 5 wt %, preferably 2 to 3 wt %tetramethylammonium hydroxide (TMAH) for 0.1 to 3 minutes, preferably0.5 to 2 minutes by a conventional technique such as dip, puddle orspray technique. In this way, a desired resist pattern is formed on thesubstrate.

EXAMPLE

Synthesis Examples, Comparative Synthesis Examples, Examples, andComparative Examples are given below by way of illustration and not byway of limitation. The average molecular weights including weightaverage molecular weight (Mw) and number average molecular weight (Mn)are determined by gel permeation chromatography (GPC) versus polystyrenestandards.

Synthesis Example 1

A 3-L flask was charged with 238.0 g of acetoxystyrene, 22.6 g of4-chlorostyrene, 189.4 g of indene, and 675 g of toluene as a solvent.The reactor was cooled to −70° C. in a nitrogen blanket, followed bythree repeated cycles of vacuum evacuation and nitrogen flow. Thereactor was warmed to room temperature, fed with 40.5 g of2,2′-azobis(2,4-dimethylvaleronitrile), V-65 (Wako Pure ChemicalIndustries, Ltd.) as a polymerization initiator, and heated at 45° C.whereupon reaction took place for 20 hours. The temperature was thenraised to 55° C. whereupon reaction took place for a further 20 hours.The reaction solution was concentrated to a half volume and precipitatedin 15.0 L of methanol. The resulting white solids were collected byfiltration and dried in vacuum at 40° C., yielding 311 g of a whitepolymer.

The polymer was dissolved again in 488 g of methanol and 540 g oftetrahydrofuran, whereupon 162 g of triethylamine and 32 g of water wereadded to the polymer solution. Deprotection reaction occurred at 60° C.for 40 hours. Then for fractionation, the reaction solution wasconcentrated and dissolved in a solvent mixture of 548 g of methanol and112 g of acetone. To this solution, 990 g of hexane was added dropwiseover 10 minutes. The mixed white turbid solution was left at rest forseparation, whereupon the lower (polymer) layer was taken out andconcentrated. The polymer concentrate was dissolved again in a mixtureof 548 g of methanol and 112 g of acetone, after which the solution wascombined with 990 g of hexane for dispersion and separation. The lower(polymer) layer was taken out and concentrated. The concentrate wasdissolved in 870 g of ethyl acetate, followed by one cycle ofneutralization, separation and washing with a mixture of 250 g of waterand 98 g of acetic acid, one cycle of separation and washing with 225 gof water and 75 g of pyridine, and four cycles of separation and washingwith 225 g of water. Thereafter, the upper layer, ethyl acetate solutionwas concentrated, dissolved in 250 g of acetone, precipitated in 15 L ofwater, filtered, and vacuum dried at 50° C. for 40 hours, yielding 187 gof a white polymer.

The polymer, designated Poly-A, was analyzed by ¹³C-NMR, ¹H-NMR and GPC,from which the composition and molecular weight were determined.

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/4-chlorostyrene/indene=76.0/6.5/17.5    -   Mw=4,200    -   Dispersity Mw/Mn=1.59

Synthesis Example 2

A 3-L flask was charged with 212.0 g of acetoxystyrene, 20.4 g of4-bromostyrene, 188.1 g of indene, and 675 g of toluene as a solvent.The reactor was cooled to −70° C. in a nitrogen blanket, followed bythree repeated cycles of vacuum evacuation and nitrogen flow. Thereactor was warmed to room temperature, fed with 40.5 g of2,2′-azobis(2,4-dimethylvaleronitrile), V-65 (Wako Pure ChemicalIndustries, Ltd.) as a polymerization initiator, and heated at 45° C.whereupon reaction took place for 20 hours. The temperature was thenraised to 55° C. whereupon reaction took place for a further 20 hours.The reaction solution was concentrated to a half volume and precipitatedin 15.0 L of methanol. The resulting white solids were collected byfiltration and dried in vacuum at 40° C., yielding 320 g of a whitepolymer.

The polymer was dissolved again in 488 g of methanol and 540 g oftetrahydrofuran, whereupon 162 g of triethylamine and 32 g of water wereadded to the polymer solution. Deprotection reaction occurred at 60° C.for 40 hours. Then for fractionation, the reaction solution wasconcentrated and dissolved in a solvent mixture of 548 g of methanol and112 g of acetone. To this solution, 990 g of hexane was added dropwiseover 10 minutes. The mixed white turbid solution was left at rest forseparation, whereupon the lower (polymer) layer was taken out andconcentrated. The polymer concentrate was dissolved again in a mixtureof 548 g of methanol and 112 g of acetone, after which the solution wascombined with 990 g of hexane for dispersion and separation. The lower(polymer) layer was taken out and concentrated. The concentrate wasdissolved in 870 g of ethyl acetate, followed by one cycle ofneutralization, separation and washing with a mixture of 250 g of waterand 98 g of acetic acid, one cycle of separation and washing with 225 gof water and 75 g of pyridine, and four cycles of separation and washingwith 225 g of water. Thereafter, the upper layer, ethyl acetate solutionwas concentrated, dissolved in 250 g of acetone, precipitated in 15 L ofwater, filtered, and vacuum dried at 50° C. for 40 hours, yielding 191 gof a white polymer.

The polymer, designated Poly-B, was analyzed by ¹³C-NMR, ¹H-NMR and GPC,from which the composition and molecular weight were determined.

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/4-bromostyrene/indene=77.7/5.4/16.9    -   Mw=4,100    -   Dispersity Mw/Mn=1.58

Synthesis Example 3

A 3-L flask was charged with 222.0 g of acetoxystyrene, 37.1 g of4-methoxycarbonylstyrene, 178.3 g of indene, and 675 g of toluene as asolvent. The reactor was cooled to −70° C. in a nitrogen blanket,followed by three repeated cycles of vacuum evacuation and nitrogenflow. The reactor was warmed to room temperature, fed with 40.1 g of2,2′-azobis(2,4-dimethylvaleronitrile), V-65 (Wako Pure ChemicalIndustries, Ltd.) as a polymerization initiator, and heated at 45° C.whereupon reaction took place for 20 hours. The temperature was thenraised to 55° C. whereupon reaction took place for a further 20 hours.The reaction solution was concentrated to a half volume and precipitatedin 15.0 L of methanol. The resulting white solids were collected byfiltration and dried in vacuum at 40° C., yielding 299 g of a whitepolymer.

The polymer was dissolved again in 488 g of methanol and 540 g oftetrahydrofuran, whereupon 162 g of triethylamine and 32 g of water wereadded to the polymer solution. Deprotection reaction occurred at 60° C.for 40 hours. Then for fractionation, the reaction solution wasconcentrated and dissolved in a solvent mixture of 548 g of methanol and112 g of acetone. To this solution, 990 g of hexane was added dropwiseover 10 minutes. The mixed white turbid solution was left at rest forseparation, whereupon the lower (polymer) layer was taken out andconcentrated. The polymer concentrate was dissolved again in a mixtureof 548 g of methanol and 112 g of acetone, after which the solution wascombined with 990 g of hexane for dispersion and separation. The lower(polymer) layer was taken out and concentrated. The concentrate wasdissolved in 870 g of ethyl acetate, followed by one cycle ofneutralization, separation and washing with a mixture of 250 g of waterand 98 g of acetic acid, one cycle of separation and washing with 225 gof water and 75 g of pyridine, and four cycles of separation and washingwith 225 g of water. Thereafter, the upper layer, ethyl acetate solutionwas concentrated, dissolved in 250 g of acetone, precipitated in 15 L ofwater, filtered, and vacuum dried at 50° C. for 40 hours, yielding 165 gof a white polymer.

The polymer, designated Poly-C, was analyzed by ¹³C-NMR, ¹H-NMR and GPC,from which the composition and molecular weight were determined.

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/4-methoxycarbonylstyrene/indene=74.9/10.0/15.1    -   Mw=4,700    -   Dispersity Mw/Mn=1.63

Synthesis Example 4

A 3-L flask was charged with 254.1 g of acetoxystyrene, 32.0 g of4-t-butoxycarbonylstyrene, 163.8 g of indene, and 600 g of toluene as asolvent. The reactor was cooled to −70° C. in a nitrogen blanket,followed by three repeated cycles of vacuum evacuation and nitrogenflow. The reactor was warmed to room temperature, fed with 39.0 g of2,2′-azobis(2,4-dimethylvaleronitrile), V-65 (Wako Pure ChemicalIndustries, Ltd.) as a polymerization initiator, and heated at 45° C.whereupon reaction took place for 20 hours. The temperature was thenraised to 55° C. whereupon reaction took place for a further 20 hours.The reaction solution was concentrated to a half volume and precipitatedin 15.0 L of methanol. The resulting white solids were collected byfiltration and dried in vacuum at 40° C., yielding 318 g of a whitepolymer.

The polymer was dissolved again in 488 g of methanol and 540 g oftetrahydrofuran, whereupon 162 g of triethylamine and 32 g of water wereadded to the polymer solution. Deprotection reaction occurred at 60° C.for 40 hours. Then for fractionation, the reaction solution wasconcentrated and dissolved in a solvent mixture of 548 g of methanol and112 g of acetone. To this solution, 990 g of hexane was added dropwiseover 10 minutes. The mixed white turbid solution was left at rest forseparation, whereupon the lower (polymer) layer was taken out andconcentrated. The polymer concentrate was dissolved again in a mixtureof 548 g of methanol and 112 g of acetone, after which the solution wascombined with 990 g of hexane for dispersion and separation. The lower(polymer) layer was taken out and concentrated. The concentrate wasdissolved in 870 g of ethyl acetate, followed by one cycle ofneutralization, separation and washing with a mixture of 250 g of waterand 98 g of acetic acid, one cycle of separation and washing with 225 gof water and 75 g of pyridine, and four cycles of separation and washingwith 225 g of water. Thereafter, the upper layer, ethyl acetate solutionwas concentrated, dissolved in 250 g of acetone, precipitated in 15 L ofwater, filtered, and vacuum dried at 50° C. for 40 hours, yielding 178 gof a white polymer.

The polymer, designated Poly-D, was analyzed by ¹³C-NMR, ¹H-NMR and GPC,from which the composition and molecular weight were determined.

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/4-t-butoxycarbonylstyrene/indene=77.8/7.0/15.1    -   Mw=5,000    -   Dispersity Mw/Mn=1.61

Synthesis Example 5

A 3-L flask was charged with 354.4 g of acetoxystyrene, 95.6 g of4-chlorostyrene, and 1500 g of toluene as a solvent. The reactor wascooled to −70° C. in a nitrogen blanket, followed by three repeatedcycles of vacuum evacuation and nitrogen flow. The reactor was warmed toroom temperature, fed with 23.6 g of AIBN (Wako Pure ChemicalIndustries, Ltd.) as a polymerization initiator, and heated at 65° C.whereupon reaction took place for 40 hours. The reaction solution wasconcentrated to a half volume and precipitated in 20.0 L of methanol.The resulting white solids were collected by filtration and dried invacuum at 40° C., yielding 420 g of a white polymer.

The polymer was dissolved again in 488-g of methanol and 540 g oftetrahydrofuran, whereupon 162 g of triethylamine and 32 g of water wereadded to the polymer solution. Deprotection reaction occurred at 60° C.for 40 hours. Then for fractionation, the reaction solution wasconcentrated and dissolved in a solvent mixture of 822 g of methanol and168 g of acetone. To this solution, 1485 g of hexane was added dropwiseover 10 minutes. The mixed white turbid solution was left at rest forseparation, whereupon the lower (polymer) layer was taken out andconcentrated. The polymer concentrate was dissolved again in a mixtureof 822 g of methanol and 168 g of acetone, after which the solution wascombined with 1485 g of hexane for dispersion and separation. The lower(polymer) layer was taken out and concentrated. The concentrate wasdissolved in 1300 g of ethyl acetate, followed by one cycle ofneutralization, separation and washing with a mixture of 375 g of waterand 98 g of acetic acid, one cycle of separation and washing with 375 gof water and 75 g of pyridine, and four cycles of separation and washingwith 225 g of water. Thereafter, the upper layer, ethyl acetate solutionwas concentrated, dissolved in 375 g of acetone, precipitated in 20 L ofwater, filtered, and vacuum dried at 50° C. for 40 hours, yielding 280 gof a white polymer.

The polymer, designated Poly-E, was analyzed by ¹³C-NMR, ¹H-NMR and GPC,from which the composition and molecular weight were determined.

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/4-chlorostyrene=75.8/24.2    -   Mw=5,200    -   Dispersity Mw/Mn=1.62

Synthesis Example 6

A 3-L flask was charged with 238.0 g of acetoxystyrene, 22.0 g of4-chlorostyrene, 190.7 g of indene, and 675 g of toluene as a solvent.The reactor was cooled to −70° C. in a nitrogen blanket, followed bythree repeated cycles of vacuum evacuation and nitrogen flow. Thereactor was warmed to room temperature, fed with 40.5 g of2,2′-azobis(2,4-dimethylvaleronitrile), V-65 (Wako Pure ChemicalIndustries, Ltd.) as a polymerization initiator, and heated at 45° C.whereupon reaction took place for 20 hours. The temperature was thenraised to 55° C. whereupon reaction took place for a further 20 hours.The reaction solution was concentrated to a half volume and precipitatedin 15.0 L of methanol. The resulting white solids were collected byfiltration and dried in vacuum at 40° C., yielding 309 g of a whitepolymer.

The polymer was dissolved again in 488 g of methanol and 540 g oftetrahydrofuran, whereupon 162 g of triethylamine and 32 g of water wereadded to the polymer solution. Deprotection reaction occurred at 60° C.for 40 hours. The reaction solution was concentrated and dissolved in870 g of ethyl acetate, followed by one cycle of neutralization,separation and washing with a mixture of 250 g of water and 98 g ofacetic acid, one cycle of separation and washing with 225 g of water and75 g of pyridine, and four cycles of separation and washing with 225 gof water. Thereafter, the upper layer, ethyl acetate solution wasconcentrated, dissolved in 250 g of acetone, precipitated in 15 L ofwater, filtered, and vacuum dried at 50° C. for 40 hours, yielding 220 gof a white polymer.

The polymer, designated Poly-F, was analyzed by ¹³C-NMR, ¹H-NMR and GPC,from which the composition and molecular weight were determined.

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/4-chlorostyrene/indene=75.6/7.5/16.9    -   Mw=4,700    -   Dispersity Mw/Mn=1.88

Comparative Synthesis Example

Polymers, designated Poly-G, Poly-H, Poly-I, and Poly-J, weresynthesized by the same procedure as in the foregoing SynthesisExamples.

Poly-G

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/indene=74.5/25.5    -   Mw=4,400    -   Dispersity Mw/Mn=1.60

Poly-H

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/4-isopropyloxystyrene/indene=73.9/11.6/14.5    -   Mw=4,100    -   Dispersity Mw/Mn=1.70

Poly-I

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/3,5-dimethoxystyrene/indene=70.8/15.6/13.6    -   Mw=4,300    -   Dispersity Mw/Mn=1.65

Poly-J

Copolymer compositional ratio (molar ratio)

-   -   hydroxystyrene/4-acetoxystyrene/indene=74.6/10.6/14.8    -   Mw=4,500    -   Dispersity Mw/Mn=1.65

The polymers synthesized are represented by the following formulae.

Examples 1 to 7 and Comparative Examples 1 to 4

Chemically amplified negative resist compositions were prepared inaccordance with the formulation shown in Tables 1 and 2. The values inTables are expressed in parts by weight (pbw). The components used inthe resist compositions and shown in Tables 1 and 2 are identifiedbelow.

TABLE 1 Example (pbw) 1 2 3 4 5 6 7 Poly-A 80 80 Poly-B 80 Poly-C 80Poly-D 80 Poly-E 80 Poly-F 80 Crosslinker 1 8.2 8.2 8.2 8.2 8.2 6.4 8.2Crosslinker 2 1.8 PAG1 8 8 8 8 8 8 8 PAG2 2 2 2 2 2 2 2 Basic compound0.4 0.4 0.4 0.4 0.33 0.33 0.33 Solvent A 320 320 320 320 320 320 320Solvent B 760 760 760 760 760 760 760 Crosslinker 1:tetramethoxymethylglycoluril Crosslinker 2: hexamethoxymethylmelaminePAG1: triphenylsulfonium 2,5-dimethylbenzenesulfonate PAG2:triphenylsulfonium 2,4,6-triisopropylbenzenesulfonate Basic compound:tris(2-methoxyethyl)amine Surfactant A: KH-20 (Asahi Glass Co., Ltd.)Solvent A: propylene glycol monomethyl ether acetate Solvent B: ethyllactate

TABLE 2 Comparative Example (pbw) 1 2 3 4 Poly-G 80 Poly-H 80 Poly-I 80Poly-J 80 Crosslinker 1 8.2 8.2 8.2 8.2 Crosslinker 2 PAG1 8 8 8 8 PAG22 2 2 2 Basic compound 0.4 0.4 0.4 0.4 Solvent A 320 320 320 320 SolventB 760 760 760 760

The resist compositions was filtered through a 0.02-μm nylon resinfilter and then spin-coated onto mask blanks having an outermost surfaceof chromium oxynitride to a thickness of 0.15 μm.

The mask blanks were then baked on a hot plate at 110° C. for 10minutes. The resist films were exposed to electron beam using an EBexposure system HL-800D (Hitachi High-Technologies Corp., acceleratingvoltage 50 keV), then baked (PEB) at 120° C. for 10 minutes, anddeveloped with a solution of 2.38% tetramethylammonium hydroxide inwater, thereby giving negative patterns.

The resulting resist patterns were evaluated as described below.

The optimum exposure dose (sensitivity Eop) was the exposure dose whichprovided a 1:1 resolution at the top and bottom of a 0.20-μmline-and-space pattern. The minimum line width (μm) of a line-and-spacepattern which was ascertained separate on the mask blank withoutcollapse when processed at the optimum dose was the resolution of a testresist. The shape in cross section of the resolved resist pattern wasobserved under a scanning electron microscope (SEM).

A cross section of the line-and-space resist pattern was also examinedfor bridge margin and undercut. The line width below which bridgesresulting from dissolution residues of the resist (i.e., resist leftundissolved in developer) are observed in spaces is reported as “bridgemargin,” with smaller values indicating better resolution in spaces.

The dry etch resistance of the resist composition following developmentwas examined by dry etching a resist film using a system TE8500S (TokyoElectron Ltd.) and observing a cross section of the resist film underSEM. A reduction in thickness of a resist film after etching isexpressed by a relative value provided that a reduction in thickness ofthe resist film of Example 5 after etching is 1.0. Smaller valuesindicate resist films with better etch resistance.

The etching was effected under the following conditions.

-   -   Prees: 250 mJ    -   RF power: 800 W    -   Gas: CHF₃ 20 sccm+CF₄ 20 sccm+Ar 400 sccm    -   Etching time: 2′30″

The resist test results are shown in Table 3.

TABLE 3 Maximum Bridge Dry Eop resolution margin etch (μC/cm²) (μm) (μm)resistance Undercut Example 1 9.2 0.06 0.06 0.89 slight 2 9.1 0.07 0.070.91 slight 3 10.5 0.08 0.11 0.92 slight 4 10.7 0.06 0.10 0.91 slight 58.9 0.06 0.07 1 slight 6 9.1 0.09 0.09 0.89 small 7 8.7 0.05 0.06 0.9slight Comparative Example 1 11.3 0.11 0.12 0.87 large 2 10.9 0.12 0.120.96 large 3 9.9 0.14 0.14 0.98 large 4 10.2 0.12 0.13 0.97 large

FIG. 1 is a photomicrograph of the resist pattern (0.10-μmline-and-space pattern) obtained in Example 1. The side walls of linesare flat and no traces of bridges are found. Even though the resist ison the chromium compound which otherwise provides strong substratedependence, only slight undercuts are seen.

FIG. 2 is a photomicrograph of the resist pattern (0.10-μmline-and-space pattern) obtained in Comparative Example 2. Some lineshave fell down due to extreme undercuts, and some lines collapsefollowing bridge formation, leaving small horn-like projections fromlines.

It is seen from Table 3 and the photomicrograph of FIG. 1 that when achemically amplified negative resist composition is prepared using ahydroxystyrene based polymer comprising electron withdrawinggroup-bearing styrene units represented by formula (2) as constituentunits, applied as a resist coating, and processed to form a pattern, thepattern profile is significantly improved even though the resist patternis on the chromium compound which is otherwise likely to invite patternprofile defectives near the substrate. Similarly the problem thatbridges form between microscopic structures is also ameliorated.

In summary, the negative resist composition of the invention is definedas comprising as a base resin a polymer which is obtained bycopolymerizing a monomer having a structure capable of converting to afunctional group providing solubility through deprotection reaction, astyrene monomer having substituted thereon an electron withdrawinggroup, typically chlorine, bromine or iodine, and optionally asubstituted or unsubstituted indene monomer, followed by deprotectionreaction. The composition offers a high contrast of alkaline dissolutionrate before and after exposure, forms a resist pattern of a satisfactoryprofile on a mask blank, especially a mask blank having an outermostsurface of transition metal compound material, which indicates a highresolution, and exhibits satisfactory etch resistance. Accordingly, thecomposition is suited as a micro-patterning material for the fabricationof VLSI and a mask pattern-forming material.

Japanese Patent Application No. 2007-087243 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A chemically amplified negative resist composition comprising apolymer comprising recurring units having the general formulae (1) and(2):

wherein R¹ and R² are each independently hydrogen or methyl, X is anelectron withdrawing group, m is 0 or an integer of 1 to 4, and n is aninteger of 1 to 5, said polymer having a weight average molecular weightof 1,000 to 50,000.
 2. The resist composition of claim 1 wherein theelectron withdrawing group represented by X has an active structuredirectly bonded to the benzene ring, the active structure being at leastone member selected from the group consisting of a halogen atom,carbonyl group, nitro group, cyano group, sulfinyl group, and sulfonylgroup.
 3. The resist composition of claim 2 wherein the electronwithdrawing group represented by X is at least one member selected fromthe group consisting of chlorine, bromine, and iodine.
 4. The resistcomposition of claim 1 wherein said polymer further comprises recurringunits having the general formula (3):

wherein R³ and R⁴ are each independently hydrogen, optionallysubstituted hydroxyl, or halogen, and u is 0 or an integer of 1 to
 5. 5.The resist composition of claim 1 wherein said polymer has a weightaverage molecular weight of 2,000 to 8,000.
 6. The resist composition ofclaim 1 wherein said polymer has a dispersity Mw/Mn equal to or lessthan 1.7.
 7. A pattern forming process comprising the steps of: applyingthe resist composition of claim 1 onto a substrate to form a coating,heating the coating, exposing the coating to light, soft x-ray orelectron beam, post-exposure heating the coating, and developing thecoating with an aqueous alkaline solution.
 8. A resist pattern formingprocess comprising providing a substrate having a surface composedmainly of a transition metal compound, providing a chemically amplifiednegative resist composition comprising a polymer comprising recurringunits having the general formulae (1) and (2):

wherein R¹ and R² are each independently hydrogen or methyl, X is anelectron withdrawing group, m is 0 or an integer of 1 to 4, and n is aninteger of 1 to 5, said polymer having a weight average molecular weightof 1,000 to 50,000, and forming a resist pattern on the substratesurface using the chemically amplified negative resist composition. 9.The process of claim 8 wherein the electron withdrawing grouprepresented by X has an active structure directly bonded to the benzenering, the active structure being at least one member selected from thegroup consisting of a halogen atom, carbonyl group, nitro group, cyanogroup, sulfinyl group, and sulfonyl group.
 10. The process of claim 8wherein said polymer further comprises recurring units having thegeneral formula (3):

wherein R³ and R⁴ are each independently hydrogen, optionallysubstituted hydroxyl, or halogen, and u is 0 or an integer of 1 to 5.11. The process of claim 8 wherein said transition metal compoundcomprises at least one transition metal selected from the groupconsisting of chromium, molybdenum, zirconium, tantalum, tungsten,titanium, and niobium, and optionally, at least one element selectedfrom the group consisting of oxygen, nitrogen and carbon.